1. Field of the Invention
The present invention relates to devices for the controlled tightening of threaded fasteners, and more particularly to tightening wrenches with angle indicating devices for the tightening of screws or bolts and nuts to a predetermined tensile stress.
2. Description of the Prior Art
In many applications of threaded fasteners, especially highly stressed clamping screws and clamping bolts, it is essential that the fasteners be subjected to a predetermined clamping preload, not only in order to avoid accidental loosening of the fastener combination, but, more importantly, in order to preclude unacceptable stress reversals in the fasteners and in the clamped parts. The establishment of a controlled fastener preload is particularly important in connection with applications where a series of fasteners is involved and where variations in the clamping tensions from fastener to fastener would not only risk the failure of certain fasteners, but also result in distortions of the parts which are clamped together.
The most common approach taken in order to achieve this result concerns itself with the application of a predetermined tightening torque to the fasteners, under the assumption that the torque applied is a reliable indicator of the tensile stress condition created in the fastener members. Accordingly, there are available in the prior art numerous suggestions of torque limiting and torque indicating devices, known under the general designation of torque wrenches, which are intended to establish a controlled predetermined tightening condition in threaded fasteners. But, no matter how precisely the devices themselves limit the torque applied to a clamping screw or nut, they cannot take into consideration differences and irregularities that exist in the relationship between the applied torque and the tensile stress created in the fastener combination by that torque. This relationship is greatly influenced by the friction conditions between the mating threads, on the one hand, and by the friction between the rotationally sliding clamping faces of the fastener and one of the clamped parts, on the other hand. Obviously, various factors may influence the actual friction conditions, chief among them being the state of lubrication and the quality of the machined surfaces, or of the applied surface coatings, for example. Small differences in lubrication, for example, may result in considerable variations in bolt tensions, even though a uniform torque was applied in all cases to the bolt head or to the nut.
It has therefore already been suggested that, in order to avoid this source of unpredictability in the tightening of threated fasteners, the latter should first be pre-tightened with a small torque, and that the final tightening to the desired tension near the yield stress of the fastener combination should be controlled on the basis of a particular angle of tightening rotation between the fastener members, rather than on the basis of a particular torque applied thereto. It has thus been suggested that such a tightening wrench for screws, or bolts and nuts include an angle indicating device with means for adjusting the starting angular position of the indicating device and means producing a reading of the angular displacement during the final tightening operation. The adjustability of the starting angular position is necessary, because it is generally accepted as impossible or impractical to eliminate the randomness of the angular relationship between the torque transmitting surfaces on the fastener members, e.g. the hexagon facets of a nut or bolt, and the threads of that part.
One prior art solution, therefore, suggests an angle indicating device for use in conjunction with a tightening wrench, where the device includes a stationary member engaging one of the clamped parts in the area surrounding the screw or nut and carrying a reference disc with angular gradations, the latter being angularly adjustable in relation to the stationary member, being frictionally held in place with the aid of permanent magnets. A hub which is solidary with a centrally located torque transmitting member carries one or several pointers which move over the gradations of the reference disc. The stationary member, when not engaged against the clamped parts, is loosely carried by the central hub of the tightening wrench. This device is disclosed in the German Pat. No. 1,603,768.
Among the shortcomings of the device just described are its structural complexity and its manufacturing cost, in addition to its considerable bulk. The device is also limited in regard to its use, being dependent upon a particular structural cooperation between the stationary member of the angle indicating device and the clamped part which is to position the former.
Another prior art solution, intended to have a greater versatility of application, suggests an angle indicating device which consists of two elements which are structurally separate from the tightening wrench itself and of which one can be attached to the wrench, while the other is intended for attachment to the clamped part or some other nearby stationary reference support. Again, permanent magnets are used to provide the necessary angular adjustability. As an alternative to attaching the pointer member to the tightening wrench, it is further suggested to use an indicator disc which slips over the hexagon profile of the bolt or nut. Such a device is disclosed in the German Auslegeschrift (Published Allowed Application) No. 2,128,348.
The above prior art device has many of the shortcomings of the earlier-described prior art device, requiring attachment of the stationary indicator member to the clamped part or to some other stationary support which, consequently, needs to be of magnetically permeable metal. The fact that the component members of the indicating device are not attached to one another and to the wrench further necessitates special care in the alignment of these parts and entails the risk of loss of a component.
Still another prior art device is disclosed in U.S. Pat. No. 2,889,729. There, the tightening wrench carries an indexing ring which is rotatably connected to the wrench head and which has an angular gradation. In the handle of the wrench is incorporated a spring-loaded pawl engaging a notch of the indexing ring as long as the torque remains below a predetermined pre-tightening limit determined by a spring-biased handle portion of the wrench. Upon reaching the pre-tightening torque, the pawl of the wrench releases the indexing ring, which is then manually held in place with a finger which engages a protruding knob of the ring, so that the subsequent angular advance of the tightening wrench is indicated by the movement of the released pawl with respect to the gradation on the indexing ring.
This device is comparatively complex in structure and accordingly costly. It requires a flat space around the head of the wrench and, for proper operation, necessitates a certain degree of skill in the manual release and positioning of the indexing ring.